In the United States and many other countries, heart disease is the leading cause of death and disability. One particular kind of heart disease is atherosclerosis, which involves the deposition of fatty material on the inside of vessel walls throughout the body (commonly called "plaque"). As the plaque collects, the artery narrows and blood flow is restricted. If the artery narrows too much, the heart muscle nourished by the artery receives insufficient oxygen and a myocardial infarction or "heart attack" can occur. Atherosclerosis can occur throughout the human body, however, it is most life threatening within the coronary vasculature.
Physicians have a wide range of tools at their disposal to treat patients with coronary artery disease. Coronary artery bypass grafts or "open heart" surgery can be performed to bypass blocked artery segments. Other, less invasive procedures are available. For example, some blockages may be dissolved by chemical treatment. Alternatively, a procedure known as percutaneous transluminal coronary angioplasty (hereinafter "PTCA") may be performed in which a catheter with an expandable section on its end is placed within the narrowed artery and inflated to compact the plaque against the vessel wall, thereby relieving the blockage.
No matter what method is used to treat coronary artery disease, it is necessary for physicians to obtain quantitative information on the condition of the vasculature within the heart. Traditionally, coronary angiography has been the method of choice. Coronary angiography involves the placement of the end of a catheter at the beginning of the coronary vasculature. A small amount of radiopaque dye is injected, and a X-ray motion picture is taken while the dye is pumped through the vessels. The physician then examines the pictures and looks for any telltale narrowing of the blood flow opacified by the radiopaque dye. By the number and degree of such narrowing, the course of treatment can be determined.
Angiography has the extreme limitation of indicating only where the blood is within the vessel; it reveals nothing of the condition of the inside of the vessel and the vessel wall itself. Furthermore, most angiography machines present virtually only one-dimensional projections of where blood flow exists. Because of this imaging limitation, the complex structures within the coronary vasculature often exhibit quite ambiguous images.
Recently, imaging of soft tissue such as gross cardiac structures has provided physicians with diagnostic images having quality that is unavailable from conventional techniques using X-ray radiation. In particular, magnetic resonance imaging (MRI) and ultrasound have become important diagnostic tools for cardiac assessment. Although MRI has the ability to image blood vessels, the image resolution is not sufficient to allow assessment of the condition of the walls of the vessel. Conventional ultrasound scanning also suffers from lack of resolution. More recently, high frequency (hence, high resolution) ultrasound has been used during open heart surgery to access the coronary arteries. This method requires the opening of the chest cavity to expose the heart surface and is hence limited in its application.
In an even more recent development, in vivo ultrasonic imaging of the human body creates the potential for access to a wealth of information regarding the condition of a patient's vasculature that is currently only at best indirectly available from other sources. The information received from in vivo imaging may be used as a diagnostic tool to help determine patient treatment, or as a surgical tool, supplementing angiography in PCTA.
In vivo ultrasonic imaging from within the heart has been described in U.S. Pat. No. 3,958,502 to Bom. In order to provide for ultrasonic imaging inside the human body, the Bom patent provides an array of small transducer elements which may be introduced into the body by way of catherization. The array of elements is excited at ultrasonic frequencies and the reflections or echos of the generated ultrasonic acoustic waves are detected by the piezoelectric properties of the transducers. Unfortunately, due to the nature of the material used for the transducers, the array of elements cannot be made small enough to allow passage into small areas such as the coronary arteries. Therefore, use of the Bom device is limited to within the heart chambers and the associated great arteries.
An additional limitation of the Bom device is the poor resolution caused by a sparse distribution of transducer elements. Piezoelectric materials of the type used by Bom (e.g., ceramics) have a practical limitation in size reduction. Because of this size limitation and the fact that the maximum resolution of the transducer array is limited by the center-to-center spacing of adjacent elements, the Bom device is inherently limited in the quality of its image resolution.
A further limitation of the Bom device is the fixed delays it provides for focusing an image. Such fixed delays do not provide satisfactory images for identification of tissue structures. For a satisfactory image, a dynamic focusing feature is needed to provide an optimal focus at a plurality of points in the imaging plane. One approach to implementing such a dynamic focusing feature is a so-called "synthetic focus" or "synthetic-aperture" approach disclosed in U.S. Pat. No. 4,325,257 to Kino et al.
For many diagnostic and therapeutic purposes, in vivo ultrasonic imaging must simulate real-time performance. To achieve diagnostic or therapeutic quality images in small cavities while maintaining real-time performance is a formidable task and one which applicants believe has not previously been attained.